The invention relates to a circuit arrangement for filtering and/or selecting single frequencies or frequency ranges, particularly of signals intended for at least an integrated circuit and/or signals generated by at least an integrated circuit, said circuit arrangement comprising at least two electric resonant circuits
with at least an inductive element and
at least a capacitive element.
In this respect, it should be generally noted that, when receiving and/or generating high-frequency signals, it is necessary to select individual frequencies or frequency ranges from a total number of received and/or generated signals. This selection is usually performed by means of filters comprising inductive elements and capacitive elements. The basic element in the filter technology is the resonant circuit which, in its simplest form, comprises a coil (inductance L) whose losses are represented by means of a equivalent resistance Rp parallel to the coil and a capacitor (capacitance C) arranged parallel to the coil. The dynamic range of such a resonant circuit oscillating at a resonance frequency fr=(2xcfx80)xe2x88x921xc2x7(Lxc2x7C)xe2x88x920.5 at a given power supply voltage and at a given drive (AC) current, normally decreases with an increasing quality factor Q (the so-called quality factor Q is a measure of the filter action and/or the selectivity of a circuit arrangement and increases linearly with the above-mentioned resistance Rp).
This reduction of the dynamic range is particularly a problem in modern methods of manufacturing integrated circuits, because such signal-receiving, signal-processing and/or signal-generating circuits must be able to receive, process and generate signals in the low voltage range so as to comply with the current technical requirements (note, for example, the multitude of electric and electronic apparatuses in the field of multimedia or also telecommunication).
It is therefore to be taken into account that the operating time of circuit arrangements implemented in electric and electronic apparatuses are subjected to increasingly stricter requirements, and certainly when portable apparatuses are concerned. The tendency, which in future may be even stronger, can be recognized that the operating voltage of integrated circuits (ICs) used in such apparatuses is increasingly further reduced, inter alia, also because of thermal grounds.
To achieve a stronger filter action, which is absolutely necessary in the present, very dense occupation of the frequency bands for suppressing interference, as well as it is used for often officially required elimination of unwanted side products when generating signals, the equivalent resistance Rp forcibly increases due to the above-mentioned linear relationship between the quality factor Q and the equivalent resistance Rp parallel to the coil (in so far as all further components of the circuit arrangement remain unchanged).
This increase of the equivalent resistance Rp has, however, its practical limit in that the dynamic range of the circuit arrangement is significantly reduced at a given fixed drive (AC) current generated by an active circuit as well as at a given fixed operating voltage, because the alternating voltage produced by the drive (AC) current may naturally be only twice the operating voltage at a maximum; with an increasing equivalent resistance Rp this voltage amplitude is already achieved at ever smaller alternating currents.
The result is that particularly but not exclusively in accumulator-powered mobile electric and electronic apparatuses in which the power supply voltages are smaller and smaller and simultaneously the requirements imposed on the dynamic range become increasingly stricter (for example, relating to the reception of weak unwanted signals in the presence of strong unwanted signals), the way of continuous increase of the equivalent resistance Rp for increasing the quality factor Q is not the object but is rather a drawback; moreover, the value of the drive (AC) current is often fixed by the working point adjustment of the control transistor so as to achieve a given collector current noise minimum or to achieve the transition frequency required for processing the signals.
A possibility of increasing the quality factor Q of a circuit arrangement of the type described in the opening paragraph is the conventional method of adding further turns to the inductive element of the circuit because in this case the mutual counter inductance thus produced between the additional turns of the coil plays a role.
This addition of further turns to the inductive element of the circuit is, however, not promising in so far as can be mathematically proved (compare H. H. Meinke/F. W. Gundlach: Taschenbuch der Hochfrequenztechnik; Springer-Verlag Berlin Heidelberg, 3rd edition 1968, pp. 183 etc., 185 etc.) so that an increase of the quality factor Q by way of increasing the number of turns of the inductive element of the circuit results in a significantly overproportional increase of the equivalent resistance Rp representing the losses of the circuit arrangement (for example, a double number of turns of the inductive element of the circuit is accompanied by an eight-fold increase of the equivalent resistance Rp representing the losses of the circuit arrangement). It has already been mentioned above that such an increase of the equivalent resistance Rp is a great drawback (apart from the fact that the capacitance of the parallel capacitive element would then only be one-fourth of the original capacitance of the capacitive element).
A circuit arrangement of the type described in the opening paragraph, in which at least a part of the above-mentioned difficulties has been taken into account, is known from U.S. Pat. No. 5,431,987. This document describes a noise filter with a first spiral electrode operating as an inductance on an insulator substrate and with a second spiral electrode also operating as an inductance via the first spiral electrode, while a dielectric layer is present between the two spiral electrodes. Due to their partial overlap, both inductances simultaneously operate as capacitors so that a capacitance is produced between the two spiral electrodes.
It is true that the known circuit arrangement provides an acceptable noise suppression in a signal transmission circuit, in a current supply circuit or the like, but the circuit arrangement disclosed in U.S. Pat. No. 5,431,987 has the drawback in so far that a satisfactory, i.e. xe2x80x9cfixedxe2x80x9d coupling of the two inductances (coupling factor k between the two inductances near its maximum value 1) cannot be realized in practice due to the different implementation of the second inductance as compared with the first inductance.
Moreover, in the circuit arrangement disclosed in U.S. Pat. No. 5,431,987, the quality factor Q, which in this case increases linearly with the root of the product of the inductance L1 of the first inductive element and the inductance L2 of the second inductive element (i.e. Q is directly proportional to (L1 xc2x7L2)0.5) suffers from the fact that the inductance L1 of the first inductive element and the inductance L2 of the second inductive element are different due to the different implementation and due to necessity of a planar layout so that the quality factor Q cannot reach its maximum value (as is known, (L1 xc2x7L2)0.5 only becomes maximal when L1 =L2).
Moreover, the known circuit arrangement has the problem that the two inductive elements are arranged on different and, consequently, differently ohmic metallization layers so that the losses are clearly increased and the quality factor Q is significantly reduced. Moreover, the known circuit arrangement cannot be realized in inherently symmetrical arrangements and, consequently, cannot be combined adequately with the differential or balanced circuit technique to be preferred in the integrated technique (to this end, the first inductive reactance and the second inductive reactance must be used pair-wise, which is not the case in U.S. Pat. No. 5,431,987).
It is an object of the invention to provide a circuit arrangement of the type described in the opening paragraph in which, at a given low power supply voltage, a dynamic range complying with the current requirements is possible, on the one hand, and has a maximal quality factor Q, on the other hand, while it should be possible to realize the circuit arrangement in inherently symmetrical arrangements as well as combine it with the differential or balanced circuit technique to be preferred in the integrated technique.
This object is achieved by the characteristic features defined in claim 1. Advantageous embodiments and essential further embodiments of the present invention are defined in the dependent claims.
In accordance with the teaching of the present invention, the fact that the resonant circuits and particularly the inductive elements are not DC coupled but (only) magnetically fixedly coupled to each other, a surprisingly wide dynamic range thereby becomes possible, on the one hand, while a high quality factor Q is obtained, on the other hand. This is due to the fact that an increase of the quality factor Q in the case of a purely magnetic coupling of the resonant circuits in no way involves an overproportional increase but only a directly proportional increase of the equivalent resistance Rp representing the losses of the circuit arrangement. For example, a double quality factor Q is accompanied by a double equivalent resistance Rp representing the losses of the circuit arrangement.
As regards the design and layout of the circuit arrangement according to the present invention, the facts based on this background essentially provide more possibilities than in conventional circuit arrangements. For example, it is advantageously possible to increase the arrangement of two resonant circuits with a magnetically fixed coupling (coupling factor k between the two inductive elements near its maximum value 1) to a larger number of resonant circuits and thus further raise the quality factor Q.
The fact that, in accordance with the teaching of the present invention, at least a part, preferably all resonant circuits of the circuit arrangement are arranged at or on the integrated circuit, particularly on only one metallization plate of the integrated circuit, having an essentially constant ohmic resistance, the two or more inductive elements are not arranged in different and, consequently, differently ohmic metallization plates. This is a significant advantage as compared with conventional circuit arrangements in so far that the circuit arrangement according to the present invention can always be implemented in the lowest ohmic, preferably upper metallization plate of the integrated circuit so that the resonant circuit losses occurring during operation of the circuit arrangement are clearly reduced and the quality factor Q is significantly increased which, inter alia, is also due to the fact that contacts, which are necessarily present in conventional circuit arrangements, with further higher ohmic metallization plates and resultant high additional losses are avoided in the present invention.
In this respect, those skilled in the art will particularly appreciate that the circuit arrangement according to the present invention can be realized in an inherently symmetrical arrangement and, consequently, combined also individually with the differential and balanced circuit technique to be preferred in the integrated technique. Another positive aspect of the circuit arrangement according to the invention is that a magnetically fixed coupling of the resonant circuits is realized which, as against the conventional circuit arrangements in the high-frequency technique, represents considerable progress in so far as these are conventionally coupled weakly (for example, the coupling degree in bandpass filters comprising two resonant circuits that are coupled together is often only in the range of a few percents).
If the present invention is to be further implemented in a very essential way, it is recommendable that at least two inductive elements each have one, and preferably more turns which are substantially concentric and/or parallel to each other. Such a winding can be realized, for example by arranging an inductive element in the form of a coil having at least one turn within a further coil also having at least one turn. An even higher degree of magnetic coupling can be achieved when the respective turns of at least two inductive elements are wound parallel to each other in a preferred form (in a sense comparable with a bifilar winding in which also two or more inductive elements are intertwined). In this way, a possibly fixed coupling and, hence, a maximal proximity of the inductive elements is achieved, which in turn results in a maximal magnetic coupling.
The invention also relates to an integrated circuit comprising at least a circuit arrangement of the type described hereinbefore.